But Delenn is a descendant of Valen... so Sinclair is her great grandpappy.

According to the original narrative breakdown for the show printed in the special Volume 15 of the scriptbooks, in the original plan for Sinclair, he would never have gone back in time to become Valen but instead would've gone forward in time with Delenn as his wife and the two of them having just had a child. Since it wasn't until at some point after the show had been rolling along for a bit that jms decided to make Valen be Sinclair gone back in time, I imagine that "The Parliament Of Dreams" was written with still that original plan in mind.